1. Field of the Invention
The present invention relates to a color toner capable of exhibiting excellent development performance for use in the electrophotographic field.
2. Discussion of Background
Recently, in the field of hard copies made by the electrophotographic process, black-and-white hard copies are now rapidly being replaced by full-color hard copies, and the market for full-color hard copies is now expanding.
In color image formation by full-color electrophotography, all colors are reproduced by using the three primary colors, that is, yellow, magenta and cyan, or four colors with the addition of black to the three primary colors.
The color image formation is generally conducted as follows. The light reflected by an original document is projected onto a photoconductive layer of a photoconductor through a color separation optical transmission filter which is in the relationship of complementary colors with the toner colors, whereby a latent electrostatic image is formed on the photoconductive layer. The latent electrostatic image thus formed is developed with a color toner, and a toner image is transferred to a toner-image-bearing member. Such a development and image-transfer process is repeated by use of other color toners to form a full-color image in such a manner that a plurality of color toners is superimposed on the identical toner-image-bearing member, with the adjustment of the registration. The thus obtained full-color images are then fixed on the toner-image-bearing member by only one time image-fixing operation.
The development system is divided into two systems. One is a two-component development system using a two-component developer comprising a toner and a carrier. The other is a mono-component development system without using a carrier, in which system a toner thin layer is formed on the surface of a development roller, and a latent electrostatic image is directly developed into a visible toner image with the toner.
In the two-component development system, a toner component and a carrier component are stirred together in a development unit so as to impart an appropriate charge quantity to the toner component.
With respect to a color toner, a binder resin with a low softening point is used for the color toner. This is because the binder resin for use in the color toner is required to completely melt at the image-fixing operation so that the fixed toner image may become a uniform thin layer and the fixed toner image may show excellent color reproduction and proper gloss. Further, in order to improve the replenishing performance of toner to the development unit and the transferring performance of toner, it is effective that an additive for improving the fluidity of toner (for example, finely-divided rigid particles) be externally added to the surface of the toner particles. However, through the vigorous stirring of the toner with the carrier in the development unit, the fluidity imparting agent externally deposited to the surface of the toner particles is easily embedded in the toner particles, especially in the color toner particles comprising a soft binder resin with a low softening point. The result is that the fluidity decreases as a matter of course, and the development performance, the transferring performance, and the replenishing performance tend to deteriorate.
On the other hand, the mono-component development system is considered to be advantageous because the carrier-free system can achieve a small-size, light-weight image forming apparatus. The mono-component development system includes magnetic mono-component development and non-magnetic mono-component development.
In the magnetic mono-component development system, a magnetic material is contained in the toner, and a magnet is incorporated in the development roller. The magnetic toner is magnetically attracted and supplied to the development roller. In the non-magnetic mono-component development system, no magnetic material is contained in the toner, and the toner is supplied to the development roller by means of a toner supply member which is disposed in contact with the development roller.
A magnetic material contained in the magnetic toner assumes a black color or other colors with strong tinting power. Therefore, it is difficult that a magnetic material be employed for the color toner so as not to impair the original color tone of the color toner. In view of the above-mentioned drawback of the magnetic material, a non-magnetic color toner is widely used in the mono-component development system.
In the non-magnetic mono-component development system, however, the replenishment performance of the toner to the development roller, and the retaining performance of the toner on the development roller are unsatisfactory as it is. In practice, the toner is forcibly pressed against the development roller, or the amount of toner retaining on the development roller is regulated by means of a blade. As a result, there easily occurs the so-called filming phenomenon that a toner film sticks to the surface of the development roller. In addition, the life of the development roller is shortened, and the charge quantity of toner becomes unstable. Namely, stable development cannot be ensured.
As previously mentioned, the color toner for use with the non-magnetic mono-component development system is required to meet not only ordinary conditions for the toner, but also the additional conditions, for example, the prevention of toner filming.
To eliminate the shortcomings of the conventional color toner, a variety of color toners are proposed.
For instance, in Japanese Laid-Open Patent Application 5-53369, there is proposed a toner comprising (1) finely-divided inorganic particles which have a specific surface area in a range of 1 to 150 m.sup.2 /g (in accordance with the BET adsorption isotherm), and are treated with a coupling agent having a saturated or unsaturated cyclic or non-cyclic organic group having 5 or more carbon atoms, and (2) finely-divided inorganic particles which have a specific surface area in a range of 160 to 400 m.sup.2 /g (in accordance with the BET adsorption isotherm) and a hydrophobic degree of 30% or more.
When the above-mentioned toner is used for a two-component developer, the charge quantity of toner is susceptible to change. When the toner is used in the mono-component development system, the charging characteristics of toner on the development roller become unstable, so that the toner tends to fall off the development roller or scatter therefrom.
A toner disclosed in Japanese Laid-Open Patent Application 6-202374 is a non-magnetic mono-component toner which comprises matrix toner particles and finely-divided inorganic particles deposited on the matrix toner particles, with the inorganic particles having an average particle size of 30 nm or more and less than 100 nm. This toner has the shortcomings in fluidity and replenishment performance.
In Japanese Laid-Open Patent Application 8-15890, there is proposed a mono-component developer comprising matrix toner particles with an average particle size of 4 to 9 .mu.m, and an external additive which includes small-size particles with an average particle size of 7 to 20 nm and large-size particles with an average particle size of 20 to 80 nm, with the small-size particles being contained in an amount of 1 to 2 wt %.
This toner has also the shortcomings that the toner tends to fall off the development roller and scatter therefrom because the charging characteristics of toner on the development roller are unstable.
Furthermore, a toner is disclosed in Japanese Laid-Open Patent Application 8-227171, which comprises matrix toner particles with an average particle size of 1 to 9 .mu.m, and an external additive. The external additive comprises finely-divided inorganic particles with an average particle size of 10 to 90 nm which have been treated to be hydrophobic, and finely-divided particles of a silicone compound with an average particle size of 30 to 120 nm which have been hydrophobically treated.
The above-mentioned toner is unstable in charge quantity when used in a two-component developer. When the toner is used as the mono-component developer, the toner tends to fall off the development roller and scatter therefrom because of the unstable charging characteristics on the development roller.
In addition, a toner disclosed in Japanese Laid-Open Patent Application 9-288369 comprises silica particles having a specific surface area of 20 to 50 m.sup.2 /g (in accordance with the BET adsorption isotherm), a pH value of 6 to 8, and a hydrophobic degree of 85% or more.
This toner has the drawbacks that the fluidity is insufficient and the replenishing performance is unsatisfactory.
There is disclosed a non-magnetic mono-component toner in Japanese Laid-Open Patent Application 9-297424. The above-mentioned toner comprises matrix toner particles, each comprising a polyester resin having a softening point (Sp) of 90 to 115.degree. C., and having such a glass transition point (Tg) as satisfies the relationship of (Sp)+110.ltoreq.4(Tg).ltoreq.(Sp)+170. Further, finely-divided particles with a specific surface area of 70 m.sup.2 /g (in accordance with the BET adsorption isotherm) are deposited on the surfaces of the matrix toner particles. However, the fluidity of the above-mentioned toner is insufficient, and the replenishment performance tends to become poor.
A toner disclosed in Japanese Laid-Open Patent Application 10-3179 comprises matrix toner particles with an average particle size of 4 to 12 .mu.m, and an externally added additive. The externally added additive comprises finely-divided inorganic particles with an average particle size of 1 to 50 nm, resin powders with an average particle size of 0.1 to 2 .mu.m, and metallic oxide particles with an average particle size of 0.3 to 3 .mu.m. The fluidity of this toner is still insufficient and the replenishment performance of the toner to the development roller is still unsatisfactory.